The invention relates to such a film employed in a liquid crystal display as a protective film, view angle enlarging film, an optically anisotropic film such as a phase difference film, a polarizing plate employing film, and a liquid crystal display.
A liquid crystal display operates at low voltage as well as low power consumption, and further, can be directly connected to an IC circuit. Specifically, it is possible to decrease its thickness. As a result, it is widely employed in word processors, personal computers, and the like, as the display. The basic structure of said liquid crystal display is such that polarizing plates are provided on the both sides of the liquid crystal cell.
In such liquid crystal displays, from the viewpoint of contrast and the like, those employing a twisted nematic liquid crystal (TN), having a twist angle of 90 degrees, have been replaced with those employing super twisted nematic liquid crystal (STN) having a twist angle of at least 160 degrees.
However, liquid crystal displays, employing STN, utilize double refraction or birefringence of the liquid crystal. As a result, the following problems which are tinted blue or yellow have occurred. viewing angle decreases, and it is difficult to produce acceptable color images in TN.
In order to overcome these problems, that is, to compensate for adverse effects due to double refraction, a technique has been proposed in which a phase difference plate is provided under the aforementioned polarizing plate. By employing this technique, the aforementioned tint problem is overcome. However, the viewing angle problem is largely unsolved. Further, a technique has been proposed in which a double refraction film is prepared in which the refractive index in the thickness direction of said film is greater than that in the vertical direction against the optical axis of double refraction, and the resultant film is employed as the phase difference plate. Still further, a technique has been proposed in which one plate having a positive double refraction value and the other plate having a negative refraction value are employed as the phase difference plate, or a multilayer exhibiting such properties is employed as the phase difference plate. Still further, as shown in Japanese Patent Publication Open to Public Inspection No. 7-218724, a polarizing plate is proposed in which at least one of its protective films is a plastic film comprised of acetyl cellulose, having the retardation value of 30 to 70 nm in the in-plane direction when measured employing light having a wavelength of 590 nm.
As methods to overcome these problems, various types of proposals have been made. For example, Japanese Patent Publication Open to Public Inspection No. 63-149624 proposes an F-STN system employing a stretched resin film, and Japanese Patent Publication Open to Public Inspection Nos. 3-87720 as well as 4-333019 proposes a method to carry out color compensation employing a film in which molecules of a liquid crystallizing polymer are subjected to twisted orientation for the purpose of decreasing its weight as well as its wall thickness while maintaining the compensation performance of a D-STN system. The phase deference compensation board of said liquid crystal display is comprised of a transparent base board, an oriented layer formed on said base board, and a liquid crystal polymer layer which is fixed in a twisted orientation state on said oriented layer.
Further, currently, as disclosed in Japanese Patent Publication Open to Public Inspection No. 7-191217, trials to improve the viewing angle of a liquid crystal cell have been carried out as compensation of the viewing angle of a TFT and TN liquid crystal display in such a manner that a discotic liquid crystal film is provided on the upper and lower surfaces of a liquid crystal cell. Said compensation board for the TN type liquid crystal display is comprised of a transparent base board, an alignment layer formed on said base board, and a liquid crystal alignment layer formed on said alignment layer in the same manner as the phase difference compensation board of the liquid crystal display described in the aforementioned Japanese Patent Publication Open to Public Inspection Nos. 3-87720 and 4-333019.
As described above, in recent years, in STN liquid crystal displays, and also in TFT and TN liquid crystal displays, demanded is an optical film having more advanced compensation performance than before. As the means to meet said demand, an optical film, on which a liquid crystal compound is coated, has been investigated.
On the other hand, techniques have been developed in which improvement of a crystal mode makes it possible to improve the viewing angle. For example, Japanese Patent Publication Open to Public Inspection No. 2-176625 discloses a liquid crystal display employing the liquid crystal cell of a vertical alignment (VA) liquid crystal mode which orients a liquid crystal compound vertically during non- application of voltage and substantially orients the same horizontally during application of voltage. The vertical alignment (VA) liquid crystal mode is characterized in having a wider viewing angle and higher speed response compared to the conventional liquid crystal mode. The trial sample of the liquid crystal display of the vertical alignment (VA) liquid crystal mode was already exhibited earlier (based on Nikkei Microdevice No.136, page 147, 1996). The liquid crystal display of the vertical alignment (VA) liquid crystal mode exhibits a wider viewing angle than conventional liquid crystal displays. However, when compared to CRTs, further improvement is required. In order to improve the viewing angle, it is considered to employ an optical compensation sheet in the same manner as the conventional liquid crystal mode.
Said VA type liquid crystal display comprises the liquid crystal cell with a vertical alignment orientation mode in which when no voltage is applied, liquid crystal molecules are oriented vertical to the orientation plate, while when voltage is applied, they are oriented parallel to the orientation plate. As a result, in said liquid crystal display, black is displayed as genuine black, contrast increases, and the viewing angle is relatively wider, compared to the TN and STN types. However, in accordance with an increase in size of a liquid crystal screen, an increase in the viewing angle has been increasingly demanded.
In order to increase the viewing angle of said VA system liquid crystal, the present inventors investigated the protective film for a polarizing plate. During the course of the investigation, it was found that in the VA type liquid crystal display, even though a film, which allows to control the retardation value in the in-plane direction as shown in Japanese Patent Publication Open to Public Inspection No. 7-218724, is employed, the resulting effects were low.
Further, Japanese Patent Publication Open to Public Inspection No. 9-90101 proposes that casting can be carried out without using chlorine based hydrocarbons as the solvent, by increasing the solvent selection range through substituting an acetyl group and a propionyl group of the specified range, and also proposes fatty acid cellulose esters having a low retardation value in both in-plane and thickness directions with the purpose such that the high contrast of liquid crystal displays such as the TFT type and FSTN type in which high contrast has been realized, is not degraded.
Consequently, the investigation regarding a protective film for a polarizing plate was further conducted. As a result, it was discovered that when a film was employed in which the retardation value (Rt value in the aforementioned Formula 1) in the thickness direction which was a value showing anisotropy in the in-plane direction as well as in the thickness direction without employing the conventional retardation value. in the in-plane direction in the viewing angle of the VA type liquid crystal display increased. Further, investigation was carried out regarding a method to increase the retardation value in the thickness direction. Thus the present invention was realized.
Further, in an optically anisotropic film prepared by providing a liquid crystal layer onto a support having a greater RT value, the inventors of the present invention discovered that advantageous performance was obtained not only in a VA type liquid crystal cell but also in a TN type liquid crystal cell. Still further, at that time, without employing a large amount of costly liquid crystalline compounds as well as without increasing the support thickness, a preferable optically anisotropic film was obtained. Accordingly, it is possible to obtain an optically anisotropic film as a less expensive, smaller and lighter part.
Further, when cellulose ester film is produced by dissolving cellulose ester and casting the resulting solution, any portion which h as been subjected to insufficient esterification tends to remain in the film as insoluble foreign matter particles. When the resulting film is employed to produce liquid crystal display elements, it has been found that the insoluble foreign matter particles cause problems such as hindering the formation of a polarizing state and forming abnormal light emission due to its difference in the refractive index from cellulose ester. Further, under normal light condition, it was difficult to detect said abnormality due to the presence of insoluble particles. In recent years, the high precision of liquid crystal displays is progressing and problems due to such foreign matter particles have increasingly been noted.
An object of the present invention is to provide a film for a liquid crystal display which enables enhancement of viewing angle without increasing the thickness, and provide a polarizing plate as well as a liquid crystal display using the same. Another object of the present invention is to provide a film for a liquid crystal display which minimizes problems due to abnormal light emission, while exhibiting excellent tearing strength as well as excellent water resistance, and a polarizing plate as well as a liquid crystal display using the same. Still another object of the present invention is to provide a film for a liquid crystal display which is less expensive and thinner, while exhibiting high performance optical anisotropy, and a polarizing plate as well as a liquid crystal display using the same.
The present invention and its embodiments will now be described.
The other embodiment of the invention is described.
1. An optical film comprised of a fatty acid cellulose ester film having an acetyl group and a propionyl group, and a retardation value (Rt value) in the thickness direction defined by Formula 1 of 60 to 300 nm.
Rt value=[(nx+ny)/2xe2x88x92nz]xc3x97dxe2x80x83xe2x80x83Formula 1
wherein nx represents the refractive index of the film in the direction parallel to the film casting direction, ny represents the refractive index of the film in the direction vertical to the casting direction, nz represents the refractive index of the film in the thickness direction, and d (in nm) represent the thickness of the film.
2. The film described in 1. above, having a retardation value in the thickness direction of 90 to 200 nm.
3. The film described in 1. above, having a retardation value in the thickness direction of 100 to 175 nm.
4. The film described in 3. above, having a retardation value (Ro value) in the in-plane direction defined by Formula 2 of no more than 10 nm.
Ro value=(nxxe2x88x92ny)xc3x97dxe2x80x83xe2x80x83Formula 2
wherein nx represents the refractive index of the film in the direction parallel to the film casting direction, ny represents the refractive index of the film in the direction vertical to the casting direction, and d (in nm) represent the thickness of the film.
5. The film described in any one of 1. through 4. above, having a thickness of 40 to 190 nm.
6. The film described in any one of 1. through 4. above, having a thickness of 60 to 190 nm.
7. The film described in any one of 1. through 4. above, having a thickness of 75 to 190 nm.
8. The film described in 1. or 2. above, which is produced by casting a solution which is prepared by dissolving in an organic solvent fatty acid cellulose esters having a degree (DSac) of acetyl substitution of 1.5 to 2.3 and a degree (DSpr) of propionyl substitution of 0.6 to 1.2.
9. A production method of an optical film in which when the film described in 8. above is produced by casting said solution on a belt or drum, the residual solvent amount is between 5 and 100 percent during peeling the film from said belt or drum.
10. In a production method of the film described in 1. above, a production method of an optical film wherein a solvent in solution, which is employed for casting, comprises a non-chlorine based solvent in an amount of at least 50 percent.
11. In a production method of the film described in 1. above, a production method of an optical film wherein the weight ratio of alcohol based solvent to the total solution, which is employed for casting, is no more than 0.3.
12. The film described in 1. above, which comprises 1 to 20 weight parts of at least one type of plasticizers comprised of phosphoric acid esters, fatty acid esters and phthalic acid esters with respect to 100 weight parts of fatty acid cellulose ester.
13. The film described in 1. above, which comprises 0.005 to 0.3 weight part of fine particles having an average particle diameter of no more than 0.1 xcexcm with respect to 100 weight parts of fatty acid cellulose ester.
14. The film described in 1. above, which comprises 0.8 to 2.0 weight parts of a UV absorbers with respect to 100 weight parts of fatty acid cellulose ester.
15. The film described in 1. above, wherein the number foreign matter particles having a size of 5 to 50 xcexcm, which are observed under a polarized light cross Nicole state, is no more than 200 per 250 mm2, and the number of foreign matter particles having a size of at least 50 xcexcm is substantially 0.
16. The film described in 15. above, wherein the number foreign matter particles having a size of 5 to 50 xcexcm, which are observed under a polarized light cross Nicole state, is no more than 200 per 250 mm2.
17. The vertical alignment type liquid crystal display which employs any one of optical films described in 1., 2., 3., 4., 5., 6., 7., 8., 12., 13., 14., 15., and 16. above.
An optically anisotropic film which comprises a fatty acid cellulose film support having an acyl group from 2 to 3 carbon atoms in the same, having thereon at least one liquid crystal orientation film as well as one liquid crystal layer, and further has a relationship represented by formula (I) between the refractive index nx in the x direction in the plane of said cellulose ester film support, the refractive index ny in the y direction, and the refractive index nz in the thickness direction:
(nx+ny)/2xe2x88x92nz greater than 0xe2x80x83xe2x80x83Formula (I)
The aforementioned optically anisotropic film wherein the retardation value (Rt value) represented by formula (II) is between 50 and 300 nm.
[(nx+ny)/2xe2x88x92nx]xc3x97dxe2x80x83xe2x80x83Formula (II)
wherein nx and ny each represent refractive indices in the x direction, and in the y direction in the plane of the cellulose ester film support; nz represents the refractive index of the film in the thickness direction; and d (in nm) represents the film thickness.
The retardation value (Rt value) represented by said formula (II) is preferably between 60 and 250 nm.
The thickness of said optically anisotropic film is preferably between 40 and 150 xcexcm.
It is preferable that the degree of acetyl substitution (DSac) of a fatty acid cellulose ester film is between 1.5 and 2.5, and the degree of propionyl substitution (DSpr) is between 0.6 and 1.2.
When a fatty acid cellulose ester film support is cast on a belt or a drum, said film support is preferably peeled from said belt or drum while exhibiting a residual solvent amount of 5 to 10 percent.
A solution, employed for casting a fatty acid cellulose ester film on a belt or drum, preferably comprises chlorine-free solvents in an amount of at least 50 percent by weight with respect to the entire solvent amount.
It is preferable that a solvent in a solution employed for casting a fatty acid cellulose ester film on a support comprises at least one alcohol-free solvent and the amount of an alcohol based solvent is 30 percent or less with respect to the total solvent amount.
A fatty acid cellulose ester film support preferably comprises at least one plasticizer selected from phosphoric acid ester derivatives, fatty acid ester derivatives, and phthalic acid ester derivatives in an amount of 1 to 20 percent by weight with respect to the fatty acid cellulose ester.
A fatty acid cellulose ester film support preferably comprises fine particles having an average particle diameter of 0.1 xcexcm or less in an amount of 0.005 to 0.3 percent by weight with respect to the fatty acid cellulose.
It is preferable that at least one liquid crystal layer be provided on a transparent film support and the Rt ratio of the liquid crystal layer to the transparent support, which is represented by the Formula 3 described below, is 1.2 or less.
Rt ratio=(Rtxe2x80x2/Rt)xe2x80x83xe2x80x83Formula (3)
wherein Rtxe2x80x2 represents the retardation value in the thickness direction of the liquid crystal layer, and Rt represents the retardation value in the thickness direction of the transparent support.
Herein, Rtxe2x80x2 is represented by the following formula:
Rtxe2x80x2=[(nxxe2x80x2+nyxe2x80x2)/2xe2x88x92nzxe2x80x2]xc3x97dxe2x80x2
wherein nxxe2x80x2 and nyxe2x80x2 each represent refractive indices in the xxe2x80x2 direction and the yxe2x80x2 direction, in the plane of the liquid crystal layer, while nzxe2x80x2 represents the refractive index in the thickness direction of the liquid crystal layer, and dxe2x80x2 (in nm) represents the thickness of the liquid crystal layer.
Rt is expressed by the following formula:
Rt=[(nx+ny)/2xe2x88x92nz]xc3x97d
wherein nx and ny each represent refractive indices in the x direction and the y direction in the plane of the transparent support, while nz represents the refractive index in the thickness direction of the film, and d (in nm) represents the thickness of the film.
The Rt ratio is preferably 0.8 or less.
The Rt ratio of the liquid crystal layer to the fatty acid cellulose ester film support, which is represented by the following Formula 4, is preferably 1.2 or less.
Rt ratio=(Rtxe2x80x2/Rt)xe2x80x83xe2x80x83Formula (4)
wherein Rtxe2x80x2 represents the retardation value in the thickness direction of the liquid crystal layer, and Rt represents the retardation value in the thickness direction of the fatty acid cellulose ester film support.
Herein, Rtxe2x80x2 is expressed by the following formula:
Rtxe2x80x2=[(nxxe2x80x2+nyxe2x80x2)/2xe2x88x92nzxe2x80x2]xc3x97dxe2x80x2
wherein nxxe2x80x2 and nyxe2x80x2 each represent refractive indices in the xxe2x80x2 direction and the yxe2x80x2 direction in the plane of the liquid crystal layer, while nzxe2x80x2 represents the refractive index in the thickness direction of the liquid crystal layer, and dxe2x80x2 (in rim) represents the thickness of the liquid crystal layer.
Rt is expressed by the following formula:
Rt=[(nx+ny)/2xe2x88x92nz)xc3x97d
wherein nx and ny each represent refractive indices in the x direction and the y direction in the plane of the fatty acid cellulose ester film support, while nz represents the refractive index in the thickness direction of the film, and d (in nm) represents the thickness of the film.
The Rt ratio is preferably 0.8 or less.
One of the uses of said optically anisotropic film is for a viewing angle compensation film for a liquid crystal display.
The liquid crystal mode is preferably either a twist nematic mode or a vertical alignment mode.
It is preferable that a liquid crystalline compound, which constitutes a liquid crystal layer, be a monomer having a chemically reactive group, and after being oriented on an alignment layer, said orientation is fixed while being hardened employing either light or heat.
The example of said liquid crystalline compound is a discotic liquid crystal.
Another example of said liquid crystalline compound is a liquid crystalline polymer.
An optically anisotropic film wherein luminescent points which are observed when two polarizing plates are provided on both surfaces of a cellulose ester film support so as to shield transmission light, the number of said points having a size exceeding 50 mm is zero per 250 mm2, and the number of said points having a size of 5 to 50 mm is 200 or less per 250 mm2.
The number of luminescent points having a size of 5 to 50 xcexcm is preferably 100 or less per 250 mm2.
It is possible to use an optically anisotropic film as a liquid crystal display being provided on both sides of a liquid crystal cell.